The present invention relates generally to the problem of linearisation in electronic amplifiers, and more particularly to the problem of controlling phase shift in these systems.
The demand for mobile radio telephone services has been increasing in recent years and has resulted in searches for ever more efficient modulation schemes. The most efficient forms of radio frequency (xe2x80x9cRFxe2x80x9d) modulation schemes are non-linear, e.g. Gaussian Minimum Shift Keying (xe2x80x9cGMSKxe2x80x9d). However, demands for extra capacity have led to research into linear modulation solutions, e.g. n/4 Shift DQPSK.
Linear modulation schemes produce greater gains in spectrum utlisation at the expense of variations in the envelope. These signals will undergo distortion when passed through non-linear RF amplifiers which results in a spreading of the spectrum beyond the allocated channel and the production of intermodulation products. Thus it is desirable to have a linear RF amplifier for linear modulated systems. However, conventional linear amplifiers are also inefficient, implying that there is also a need for linear amplifiers being power efficient so as to be able to power them using the batteries in mobile telephones.
It is known to use feed-forward linearisation as one method of linearising non-linear amplifiers. It is based on cancelling the distortion of the amplifier at the output. The distortion signal, or error signal, is measured by comparing the amplifier output signal with the input. This error signal is out of phase with the distortion and is applied to the output, thereby resulting in a reduction in the distortion. The error signal needs to be amplified by a linear RF power amplifier.
However, as the efficiency of an RF power amplifier increases so does its distortion, and hence the error signal level to be amplified. The larger the error signal the larger the linear amplifier and hence the greater the power consumption and the lower the efficiency. Such systems have been applied particularly for wideband systems. In short, an example of a feed-forward linearisation system may have two loops of which a first loop includes a main amplifier path that needs to have the same gain and phase shift as a reference path in order to subtract distortion created in the main power amplifier. The same applies to a second loop where an error amplifier path needs to have the same gain and phase shift as the main path so that the error can be subtracted from the error contained in the main power amplifier signal.
It is possible to achieve the control mentioned above in either the polar or the Cartesian domain. According to one method the phase and gain are controlled in the polar domain. According to another method the control is performed in the Cartesian domain where the gain and phase are controlled with the help of orthogonal vectors. Both of these methods have their particular advantages and disadvantages. A particular disadvantage of the Cartesian system is its lack of stability.
It is also known to use feedback to linearise non-linear systems. One method is to use Cartesian feedback, which uses negative feedback of the baseband quadrature modulation to provide reduction in intermodulation distortion with low complexity and cost. Cartesian gain and phase control is described in U.S. Pat. No. 5,157,346. The solution described in this patent still suffers from the stability problem. The reason for this is that the phase shift in the amplifier path and the reference path have to be equal to within at least 90 degrees or the system will be unstable. Even so, a phase error of a few degrees will still severely degrade the performance of the system. This means that a careful adjustment is needed, but temperature and ageing effects can limit the performance and even bring the system into oscillation.
Accordingly it can be seen that there still exists a need to provide linearisation of power amplifiers by using feedback in the Cartesian domain that can hold the phase error stable to within a few degrees.
As can be seen above, there still exists a problem in systems for linearisation of power amplifiers, and especially in systems which use Cartesian feedback techniques for linearisation. Present systems suffer from the problem that their phase is unstable, which leads to severe system degradation.
Accordingly, it is an object of the present invention to provide a method and apparatus for Cartesian feedback to stabilise the phase error of an amplifier to within a few degrees.
The present invention achieves the above objectives by automatically controlling the phase in a system. Feedback systems commonly have an operational amplifier. An internally compensated operational amplifier is approximately an integrator giving 90 degrees of phase shift so an additional phase shift of another 90 degrees will make the amplifier oscillate; even phase shifts in excess of approximately 30 degrees will deteriorate the performance with gain and noise peaking. If you introduce a delay or a phase shift in a loop with the operational amplifier the system is not necessarily stable. In radio technology these delays can be e.g. up- and down-converters or a power amplifier. In order to make the system stable, a corresponding phase shift has to be subtracted somewhere in the closed loop. A rotation is necessary to maintain stability under all conditions, although this can, at least in theory, be made manually by careful trimming. Typically the phase shift is several turns in a transmitter, say 20 times 360 degrees, and has to be controlled within say +xe2x88x9230 degrees.
The present invention automatically controls the phase in a system by measuring the phase difference, directly or indirectly, and then performing a rotation on the Cartesian system accordingly to adjust the phase error within a few degrees.
There are several ways to measure the phase difference. The most straightforward way is to measure the outgoing phase of the Cartesian phase and gain control element and compare this with the phase of the reference signal. Alternatively, it is possible to indirectly measure the outgoing phase by using the control signals to the gain and phase controller as a measure of the outgoing phase. This phase is then subtracted from the phase of the reference signal obtained with a Cartesian phase detector and the phase difference is used to rotate the co-ordinates.
The co-ordinate rotation can be performed at several places in the system. The rotation can be done in RF somewhere in the main amplifier path or the control signals to the gain and phase controller can be rotated. Other possibilities are phase rotation of reference signal, rotation of the signal from the main amplifier going to the summation point or rotation of the error signal.
The present invention is superior to all other methods of making this form of Cartesian loops stable and eliminates the need for cumbersome and time-consuming adjustment. Further, since the phase control is very accurate, this invention can give higher bandwidth and overall performance than any existing solutions.
Although the invention has been summarised above, the method and the system according to the invention are defined in the appended claims.